Decision Table Based Testing

Assume that a rational number is stored as follows: {Unsigned int numerator, denominator; int signed} Assume that a program has been written to add two rational numbers, provided that they are valid. It displays the results as 15/78 (for e.g.) or -15/78 as the case may be. What will the decision table for it look like? and what will be the necessary test cases for decision table based testing?